Use of conjugate fibers of a sea-island type or split type has conventionally been known as a technique for obtaining microfibers.
A method of obtaining a sea-island type conjugate fiber is to spin a combination of two or more ingredients. Removing one component of the resultant sea-island type conjugate fiber by dissolution gives microfibers. Although this technique can yield exceedingly fine fibers, it is not economical because one component is removed by dissolution.
On the other hand, a method of obtaining a splittable conjugate fiber is to spin a combination of two or more resins. The splittable conjugate fiber obtained is split into many fibers by applying a physical stress thereto or utilizing, e.g., a difference in contraction with a chemical between the resins. Thus, microfibers are obtained.
Known splittable conjugate fibers include those comprising two different polyolefins such as, e.g., disclosed in Patent document 1. The publication discloses a conjugate fiber comprising at least two polyolefin components and having a hollow in the center of the fiber in a cross-sectional configuration, in which the components are arranged radially and alternately, wherein a hollow ratio of the hollow is 5 to 40%, and a ratio of an average length W of an outer arc of the fiber to an average length L of from the hollow to the outer edge of the individual segments (W/L) is 0.25 to 2.5. The publication describes the splittable conjugate fiber as having excellent splittability. However, a polyolefin generally has a low melting point so that the polyolefin conjugate fiber is difficult to process and use at 160° C. or higher.
Patent document 2 discloses a splittable conjugate fiber, in which a polyester and a polyolefin are radially and alternately arranged into 8 or more segments in a cross-sectional configuration, which is easily splittable into microfibers providing nonwoven fabric with excellent softness and texture. The splittable conjugate fibers comprising the polyester and the polyolefin are easy to process and use at 160° C. or higher. However, when a loose aggregate of such splittable conjugate fibers, which is called a web, is subjected to physical impact such as high pressure water jets being generally conducted for splitting the splittable fibers as described in the publication, the fibers are liable to be shunted around a point of impact, resulting in easily forming holes or poor texture of the nonwoven fabric.
To address this problem, an approach has been developed, for example, when a nonwover fabric is produced using splittable conjugate fibers by an air-laid web method, the splittable conjugate fibers are blended with an ordinary olefin-based fiber as a binder fiber to thermally bond (fix) the splittable fibers via the binder fiber before application of physical impact for splitting.    Patent document 1: Japanese Patent 3309181    Patent document 2: JP-A-2000-110031